Tobacco use behavior is complex and changing, with electronic cigarettes (e-cigarettes) increasing in use. In the face of these changes, we need to know more about the behavioral phenomenon of e-cigarette use and its association with other factors, such as continued combustible tobacco use (conventional cigarettes), nicotine dependence, or changes in patterns of tobacco product use, including reductions or cessation of conventional cigarettes and uptake of e-cigarettes. Understanding e-cigarettes, as they are actually used, will help guide the FDA in their regulatory decisions. This project will use Ecological Momentary Assessments (EMA) to gather real-time reports of dual product (both conventional and e-cigarette) users' daily experiences and tobacco use to examine how the immediate context of tobacco use, along with the individual's subjective reactions to those experiences, varies by product and individual characteristics, and influences future patterns of conventional and e-cigarette use. We address several FDA research priorities: 1) the diversity of tobacco use behavior and associated attitudes, perceptions, and subjective experiences; 2) how subjective experiences associated with the use of diverse tobacco products are associated with tobacco dependence and changes in dependence; and 3) how awareness of e-cigarette messages is associated with expectancies and experiences surrounding e-cigarette use. We will recruit 450 adult cigarette smokers who also use or who are at high risk for using e-cigarettes and conduct two longitudinal waves of EMA. Our aims include: 1) examining the micro-contexts of tobacco use and how they vary by product and individual differences (e.g., demographics, tobacco history, dependence) in order to understand better the functional value of e-cigarettes; 2) examining real-time withdrawal, cravings, and satisfaction with tobacco products and how these affect transitions in tobacco use; and 3) examining how proximal tobacco cues relate to tobacco use experiences, contexts, and patterns of use. Our in-depth examination of the real-time reactions to the use of e-cigarettes directly addresses FDA questions of interest.